1. Field of the Invention
This invention relates to a circuit for conference telephony. Particularly, this invention relates to a circuit for conference telephony which, in the digital signal transmission system having serial data transmitted and received by the use of packets, permits synthesis of voices.
2. Description of the Prior Art
In recent years, the development of local area networks (LAN) of the class represented by ETHERNET (Japanese Patent Publication SHO No. 57(1982)- 55264) has come to gather momentum. The LAN adopts a system wherein the digital signals transmitted on a communication cable are located within frames which are major enclosures periodically repeated on a time axis and a plurality of stations occupy regions on the time-sharing basis within the frames.
A type of conference telephony using the local area network is designed to be operated by a hybrid (hereinafter abbreviated as "HYB") device.
In the conventional conference telephony, an A station 1 which convenes a conference first establishes a call with a HYB station 2 and then calls up a B station 3 as shown in FIG. 1A. Then, the B station 2 establishes a call with the HYB station 2. Subsequently, as shown in FIG. 1 (b), the A station 1 calls up a C station 4, a D station 5, . . . sequentially and the C station 4, the D station 5, . . . so called up sequentially establish a call with the HYB station 2.
Consequently, calls are established as radially connected between the HYB station 2 at the center and the individual stations as shown in FIG. 1C. The HYB station 2 synthesizes the voice packets from the stations and sends the product of synthesis back to the stations.
The conference telephony has been heretofore materialized by a procedure generally of the foregoing description.
The conventional system of this nature, however, has the disadvantage that it requires a HYB station furnished with a HYB apparatus adequate for the number of members taking part in the conference and, when the number of members is limited, the system cannot be fully utilized and proves costly. This system also has the possibility that a trouble in the HYB station will induce general disruption of the services of all the stations.